A compression encoding technology is used to efficiently transmit and store moving picture data. For example, as moving picture technology, MPEG-1, 2, 4, and H.261 to H.264 standards are widely used. In such moving picture encoding technology, a prediction signal of a target image serving as an encoding target is produced by using adjacent images side by side on a temporal axis and then a difference between the target image and the prediction signal is encoded, thereby realizing a data amount reduction. This technique is called inter-frame encoding.
For example, in an encoding method specified by the H.264 standard, one frame image is divided into block regions each composed of 16×16 pixels, and the image is encoded on the block-by-block basis. In the inter-frame encoding method, a prediction signal is produced by carrying out motion compensating prediction on a target block of an encoding target image with reference to other frames that have been encoded and restored. Then, a differential value of the target block and the prediction signal is obtained. The differential value is discrete-cosine-transformed and quantized so as to produce encoded data.
Besides the inter-frame encoding method, a bidirectional prediction method is used to reduce quantized noises included in prediction signals and predict a newly appearing image signal. In the bidirectional prediction method, two pieces of motion amount are set to a target block. First motion amount determines a first prediction signal from a first reference image that is a temporally past image relative to the target block, while second motion amount determines a second prediction signal from a second reference image that is a temporally future image relative to the target block. The first and second prediction signals are averaged to produce a prediction signal. Alternatively, the first and second prediction signals may be obtained from two respective reference images that are temporally past images, and averaged to produce a prediction signal.